The math
Concrete is sold by the cubic yard. Volume is just length × width × thickness, but with thickness usually in inches and length/width in feet, getting the units right is the only place this calculation goes wrong.
slab_yd³ = (length_ft × width_ft × thickness_in ÷ 12) ÷ 27
column_yd³ = (π × (diameter_in / 24)² × height_ft) ÷ 27
The ÷ 27 converts cubic feet to cubic yards. A cubic yard is 3 ft × 3 ft × 3 ft = 27 ft³.
Frequently asked
How much overage should I add?
Standard is 5–10%. Add more for: rough or unlevel ground, formwork that's not perfectly square, anywhere the pour might over-fill (uneven base), and crews that can't pour quickly enough to manage a tight batch. Pros routinely order 10% over.
Bag count math
A 60 lb bag of concrete makes about 0.45 ft³ of finished concrete. An 80 lb bag makes about 0.60 ft³. To convert: bags = total_ft³ ÷ ft³_per_bag. For anything over a few yards, the bag math will look painful, and it's accurate.
What about rebar and prep?
Volume calculations ignore reinforcement steel, because rebar takes up negligible space. They also don't account for base preparation (gravel, sand, vapor barrier). Plan those separately.